Refraction by Spherical Lenses

Lens Formula

$$\frac{1}{v} - \frac{1}{u} = \frac{1}{f}$$

Power of a Lens

$$P = \frac{1}{f \, (\text{in metres})}$$

The SI unit of power is the dioptre ($\text{D}$).

Example

A convex lens has a focal length of $20\,\text{cm}$. Find its power.

$$f = 20\,\text{cm} = 0.20\,\text{m}$$ $$P = \frac{1}{0.20} = +5\,\text{D}$$